1. Field of the Invention
The present invention relates to light emitting devices comprising light emitting diode (LED) assemblies and LED array assemblies and methods of fabrication thereof. More especially, the invention concerns LED lens assemblies and LED array lens assemblies. Moreover, although not exclusively, the invention concerns LED assemblies with phosphor enhancement.
2. Description of the Related Art
Achieving the brightness expected from today's light emitting diodes (LEDs) requires efficiently extracting the light generated by the LED chip/die. LED chips typically have either an edge-emitting or surface-emitting structure. Total internal reflection is the major loss mechanism that reduces the proportion of emitted photons and as is known the total internal reflection of photons incident to surface at angles greater than a critical angle θc defined by Snell's law:θc=sin−1(n0/n1)where n0 and n1 are respectively the indices of refraction for air and LED chip material. In addition absorption within the LED material and Fresnel reflection loss at the interface reduce the light output efficiency of the LED. The total efficiency for electrical to optical conversion in a flat-surface LED is given by [W. N. Carr and G. E. Pittman, Appl. Phys. Lett. 3, 173(1963)]:
      η    F    ≈                    4        ⁢                  n          0                ⁢                  n          1                                      (                                    n              0                        +                          n              1                                )                2              ⁢          (              1        -                  cos          ⁢                                          ⁢                      θ            C                              )      where 4 n0n1/(n0+n1)2 is the transmission coefficient and (1−cos θc) is the solid cone.
To increase extraction efficiency, a stepwise reduction in the index of refraction from the chip to air may be employed to reduce the total internal reflection loss. For example if a medium with an intermediate index of refraction n2=√{square root over (n0n1)} is used to encapsulate the LED chip, then the critical angle between LED chip and medium is increased to θc=sin−1(√{square root over (n0/n1)}), resulting in a factor of (n2/n0)2=n1/n0 increase in the extraction efficiency relative to air plus some additional correction for Fresnel reflection losses.
Other techniques have been used to increase extraction efficiency. For example roughing the LED chip surface is used to effectively enlarge the surface area of the chip and to thereby ensure that more rays of emitted light fall within the critical angle zone provided by the effectively enlarged surface area. Another technique involves using a photonic crystal to couple as many modes of the emitted light as possible, especially those in the lower orders, out of the top surface of an LED chip.
In addition, optical components such as convex lens are commonly used to alter the emitted radiation pattern of an LED chip to collimate its emission to a desired direction and enhance the brightness of the illumination in that direction. Commonly the lens is surface mounted on an individual LED chip. Moreover, a surface mounted lens made of glass or a plastics material also enhances light extraction since its index of refraction is lower than that of chip material and greater than that of air. In addition, its convexly curved surface considerably increases the size of critical angle zone between the lens and air which assists in extracting more amount of light from the underlying chip. A problem with mounting such lenses is that due to the size of the LED chip, which is typically a few microns, the tolerances for aligning the lens with the chip are very precise and the process can become expensive. For this reason the LED chip or a number of chips are often mounted within a container and the lens mounted to the container.
The recent development of white LEDs based on an InGaN (indium gallium nitride) blue LED chip and various yellow phosphors (photo-luminescent materials) to make an LED that emits light which appears white in color to the eye have made possible many technological and commercial applications including solid state lighting. At present particles of the phosphor material are incorporated in an encapsulating material which is then applied to individual LED chips, which have been diced and packaged. Following the application of the phosphor the lens is surface mounted to the encapsulating phosphor layer. Such a fabrication technique is inefficient, time consuming and expensive.
The inventors have appreciated that the application of phosphor material directly onto the surface of, or embedding it inside, a lens or some other such optical components could simplify the manufacturing process and may in the case of a white LED achieve a better white light uniformity. However, individually packaging phosphor materials along with lenses at discrete chip level is still a time-consuming process.
A need exists therefore an improved method of fabricating LED lens assemblies which can incorporate phosphor enhancement at a wafer level in order to maintain manufacturing quality consistency, improve packaged chip performance, and achieve higher yield rates.